High pressure diaphragm valves with an exchangeable seat assembly are e.g. used as gas cylinder valves or as shut-off valves for high pressure toxic, corrosive, highly oxidizing or highly flammable gases, but also for high pressure inert gases. They show an excellent gas tightness and resistance to such high pressure gases.
Such a high pressure diaphragm valve with an exchangeable seat assembly is e.g. disclosed in U.S. Pat. No. 5,215,286. It comprises a valve body having therein a valve chamber of a generally cylindrical configuration. This valve chamber has at one end a flat bottom surface with a central gas inlet port and a lateral gas outlet port therein. A flexible multi-layer diaphragm seals the opposite end of the valve chamber. A seat assembly, including a metal mounting ring and synthetic seat ring, is removably fitted in the valve chamber. The mounting ring has a central, cylindrical seat hole, an outer ring flange and a plurality of circumferentially spaced through openings arranged between the seat hole and the outer ring flange. The synthetic seat ring is fitted in the central seat hole of the mounting ring. It is a cylindrical body with a ring flange at its foot end and a central through hole. The foot end with the ring flange forms a seat ring foot surface that is pressed by the mounting ring onto the flat bottom surface around the central gas inlet port, wherein an inner flange of the mounting ring overlies the ring flange of the synthetic seat ring and compresses the latter. The other end of the synthetic seat ring forms a funnel-shaped seat surface in the valve chamber. A thin-walled rigid metal sleeve is closely received within the seat ring and rests with one end on the bottom surface of the valve chamber, wherein the funnel-shaped seat surface protrudes over the other end of the metal sleeve. The rigid metal sleeve and the mounting ring closely engage about the interior and exterior of the seat ring to prevent radial deformation and axial deflection of the latter when the seat surface is engaged by the diaphragm. With the help of a sleeve or bonnet member, the outer rim of the diaphragm is clamped about its periphery to an upwardly extending sealing bead on the outer ring flange of the mounting ring, whereby the outer ring flange of the mounting ring is simultaneously pressed with a downwardly extending sealing bead onto the bottom surface of the valve chamber. Between the mounting ring and the flat bottom surface of the valve chamber remains an annular gas collecting space that is radially delimited by the ring flange of the synthetic seat ring and the downwardly extending sealing bead of the mounting ring. When the valve is open, gas flows through the central gas inlet port, traverses the seat ring through the metal sleeve, flows over the seat surface of the seat ring, enters through the through openings in the mounting ring into the annular gas collecting space, to finally enter into the lateral outlet port, which opens into this annular gas collecting space. Actuating means, including a push rod and an actuator button, allow to selectively deflect the diaphragm into engagement with the seat surface, so as to close the valve.
It will be noted that the valve disclosed in U.S. Pat. No. 5,215,286 has however several drawbacks. It will for example be appreciated that mounting a thin rigid metal sleeve into a through-hole of a synthetic seat ring is not very easy, in particularly not within the context of an automated production process of the seat assembly. Furthermore, a metal sleeve that is not well fixed in the synthetic seat ring may damage the synthetic seat ring or the diaphragm when the valve is closed or it may be ejected into the valve chamber, if a high velocity gas stream passes through the central gas inlet port. It will also be appreciated that, gas tightness of this prior art valve at very high pressures is largely dependent on the gas tightness that is achieved between the foot surface of the seat ring and the bottom surface around the central gas inlet port.
The aforementioned drawbacks are partially overcome with an exchangeable seat assembly as disclosed in EP-A-1281898. In this prior art valve, the metal sleeve in the seat ring is replaced by a protruding thin rim portion formed directly on the valve body. This rim portion and the mounting ring define a recess in which the synthetic valve seat is received. The bottom surface of the valve chamber includes a seat supporting surface surrounding the rim portion and a recessed channel surrounding the seat supporting surface. One end of the mounting ring is fitted into the recessed channel where it bears on the annular bottom surface of the latter. An annular recess in the surface of the mounting ring facing the annular bottom surface of the recessed channel serves as gas collecting space. The inner flange of the mounting ring overlies the ring flange of the synthetic seat ring and presses the latter onto the seat supporting surface.
It has to be pointed out that the valve disclosed in EP-A-1281898 still has e.g. the following drawbacks. First, one has to respect relatively narrow manufacturing tolerances for the recessed channel in the valve body, the seat support surface, the seat ring and the mounting ring, in order to achieve the required gas tightness without damaging the synthetic seat ring. Second, the annular recess in the lower surface of the mounting ring, which serves as gas collecting channel, substantially weakens the mounting ring, which may result in deformations of the mounting ring affecting gas tightness and possibly causing the whole valve to be ruined. Third, the ring flange of the seat ring is easily damaged during assembly of the valve and when the valve is subjected to excessive closure forces. Fourth, the seat ring too risks to be damaged when the valve is subjected to excessive closure forces. Fifth, the design of the valve seat is not really adapted for withstanding adiabatic shock tests with high pressure oxygen.